Multi-piece solid golf ball

ABSTRACT

A multi-piece solid golf ball has a core, at least one intermediate layer encasing the core, and a cover of at least one layer encasing the intermediate layer. The core is formed of a base rubber, and the intermediate layer and cover are each formed of a resin material. The intermediate layer has a thickness (a) and the cover has a thickness (b) such that the ratio a/b is from 0.7 to 1.9, and the core has a diameter (c) such that the ratio c/a with the intermediate layer thickness (a) is from 23 to 38. The intermediate layer has a material hardness (Shore D) of from 42 to 76, and the cover has a material hardness (Shore D) of from 41 to 69. The ball satisfies the following relationship: cover material hardness&lt;intermediate layer material hardness&gt;core surface hardness, and wherein a Shore D hardness at the core surface minus a Shore D hardness at the core center is not more than 17 units. This golf ball has an improved flight performance and a good, solid feel on impact.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending application Ser. No. 12/975,018 filed on Dec. 21, 2010, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a multi-piece solid golf ball composed of a core, an intermediate layer and a cover that have been formed as successive layers. More specifically, the invention relates to a multi-piece solid golf ball which has an improved flight performance and a good, solid feel on impact.

Numerous golf balls with a three-piece construction that includes, as mentioned below, an intermediate layer situated between a core and a cover, the respective layers having specific hardnesses and thicknesses, have hitherto been disclosed as solid golf balls which address the needs of professional golfers and skilled amateurs. Some of these prior-art golf balls also possess improved spin properties, flight performance and durability.

U.S. Pat. No. 6,632,149 discloses a golf ball having an intermediate layer/cover hardness relationship which is soft/hard, and in which the intermediate layer is thinly formed. U.S. Pat. No. 4,109,778 discloses a golf ball in which the intermediate layer/cover hardness relationship is soft/hard, and which has an optimized core hardness profile. U.S. Pat. No. 4,045,089 discloses a golf ball in which the intermediate layer/cover hardness relationship is hard/soft, and wherein the intermediate layer is thin, having a thickness of less than 1 mm. U.S. Pat. No. 4,247,030 discloses a golf ball in which the intermediate layer/cover hardness relationship is soft/hard, and which uses polyurethane as the cover material. U.S. Pat. No. 2,910,516 discloses a golf ball in which the cover has been formed so as to be relatively thicker than the intermediate layer. U.S. Pat. No. 3,661,812 discloses a golf ball in which the hardness difference between the core surface and the intermediate layer is optimized, the intermediate layer/cover hardness relationship is hard/soft, and which has a modified dimple design. U.S. Pat. No. 3,516,125 discloses a golf ball in which the cover is composed primarily of polyurethane, the intermediate layer/cover hardness relationship is hard/soft, and which has a modified dimple design. U.S. Pat. No. 3,601,582 discloses a golf ball which has a suitable core deflection under predetermined loading, a suitable intermediate layer hardness and a suitable cover hardness, and in which the dimple trajectory volume has been quantitatively optimized.

However, the degree of improvement achieved in these golf balls remains inadequate. In particular, there exists a desire for improvements that strike a good balance between a lower spin rate, an increased distance on shots with a W#1, and a good feel on impact.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a multi-piece solid golf ball which is endowed with both an excellent distance and a good, solid feel on impact.

The inventors have conducted extensive investigations in order to attain the above object. As a result, they have discovered that, in a multi-piece solid golf ball having an intermediate layer situated between a core and a cover, by forming the core of a base rubber, forming the intermediate layer and the cover each of a resin material, setting the ratio a/b between the intermediate layer thickness (a) and the cover thickness (b) within a given range, setting the ratio a/c between the intermediate layer thickness (a) and the core diameter (c) within a given range, and optimizing the material hardness of the intermediate layer and the material hardness of the cover, it is possible to improve the ball rebound and improve the spin rate-lowering effect on shots with a driver, thereby increasing the distance traveled by the ball, in addition to which a good, solid feel can be obtained on shots with a driver.

Accordingly, the invention provides the following multi-piece solid golf ball.

-   [1] A multi-piece solid golf ball comprising a core, at least one     intermediate layer encasing the core, and a cover of at least one     layer encasing the intermediate layer, wherein the core is formed of     a base rubber, the intermediate layer and cover are each formed of a     resin material, the intermediate layer has a thickness (a) and the     cover has a thickness (b) such that the ratio a/b is from 0.7 to     1.9, the core has a diameter (c) such that the ratio c/a with the     intermediate layer thickness (a) is from 23 to 38, the intermediate     layer has a material hardness (Shore D) of from 42 to 76, the cover     has a material hardness (Shore D) of from 41 to 69, and the ball     satisfies the following relationship: cover material     hardness<intermediate layer material hardness>core surface hardness,     and wherein a Shore D hardness at the core surface minus a Shore D     hardness at the core center is not more than 17 units. -   [2] The multi-piece solid golf ball of [1] wherein, letting the ball     deflection (mm) when compressed under a final load of 490 N (50 kgf)     from an initial load state of 98 N (10 kgf) be A and letting the     ball deflection (mm) when compressed under a final load of 5,880 N     (600 kgf) from an initial load state of 98 N (10 kgf) be B, the     value of B/A×100 is from 830 to 930. -   [3] The multi-piece solid golf ball of [1], wherein the ball     deflection B is from 7.0 to 10.0 mm, and the core deflection when     compressed under a final load of 1,275 N (130 kgf) from an initial     load state of 98 N (10 kgf) is from 2.1 to 4.1 mm. -   [4] The multi-piece solid golf ball of [1], wherein the resin     material of the cover is formed by injection molding a single resin     blend composed primarily of (A) a thermoplastic polyurethane and (B)     a polyisocyanate compound, which resin blend includes a     polyisocyanate compound in at least some portion of which all the     isocyanate groups on the molecule remain in an unreacted state. -   [5] The multi-piece solid golf ball of [1], wherein a plurality of     dimples are formed on a surface of the ball, the total number of     dimples is from 250 to 350, the dimples have a surface coverage (SR)     of at least 75%, and the ball, when hit, has a coefficient of lift     CL of the ball at a Reynolds number of 70,000 and a spin rate of     2,000 rpm which is at least 60% of the coefficient of lift CL at a     Reynolds number of 80,000 and a spin rate of 2,000 rpm. -   [6] The multi-piece solid golf ball of [5] which uses at least five     types of dimples of differing diameter and/or depth, and which     includes from 6 to 30 small dimples having a diameter of 3.0 mm or     less.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic sectional view showing a multi-piece solid golf ball (three-layer construction) according to the invention.

FIG. 2 shows a dimple pattern used in a ball according to one embodiment of the invention, (A) being a top view and (B) being a side view.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below.

The multi-piece solid golf ball of the present invention, as shown in FIG. 1, is a golf ball G having a core 1, an intermediate layer 2 encasing the core, and a cover 3 encasing the intermediate layer. The cover has a plurality of dimples D formed on the surface thereof. The core 1, intermediate layer 2 and cover 3 are each not limited to a single layer, and may be formed of a plurality of two or more layers.

In the invention, the core has a diameter which, although not subject to any particular limitation, is preferably at least 35.7 mm, more preferably at least 36.7 mm, and even more preferably at least 37.7 mm. The diameter upper limit, although not subject to any particular limitation, is preferably not more than 41.7 mm, more preferably not more than 40.7 mm, and even more preferably not more than 39.7 mm. At a core diameter outside this range, the ball may have a lower initial velocity or a poor feel on impact.

The core has a center hardness which, although not subject to any particular limitation, has a Shore hardness value of preferably at least 35, more preferably at least 36, and even more preferably at least 37. The upper limit, although not subject to any particular limitation, is preferably not more than 54, more preferably not more than 53, and even more preferably not more than 52. If this value is too small, the rebound may be inadequate, as a result of which a sufficient distance may not be achieved. On the other hand, if this value is too large, the spin rate of the ball on full shots by a driver may become too high, as a result of which a sufficient distance may not be achieved. And, the feel of the ball may become hard.

On the other hand, the core has a surface hardness which, although not subject to any particular limitation, has a Shore hardness value of preferably at least 45, more preferably at least 46, and even more preferably at least 47. The upper limit, although not subject to any particular limitation, is preferably not more than 73, more preferably not more than 68, and even more preferably not more than 66. If this value is too small, the rebound may be inadequate, as a result of which a sufficient distance may not be achieved, and the ball may have a poor durability to cracking on repeated impact. On the other hand, if this value is too large, the feel of the ball on full shots may become hard and the spin rate may become too high, as a result of which a sufficient distance may not be achieved.

In the present invention, it is critical that a Shore D hardness at the core surface minus a Shore D hardness at the core center is not more than 17 units. The upper limit of the above value is preferably not more than 16 and more preferably not more than 15. If this value is too great, the ball on full shots may deform largely to yield an initial velocity loss of the ball, as a result of which a sufficient distance may not be achieved.

The core has a deflection under loading, i.e., a deflection (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), which, although not subject to any particular limitation, is preferably at least 2.1 mm, more preferably at least 2.4 mm, and even more preferably at least 2.7 mm. The upper limit, although not subject to any particular limitation, is preferably not more than 4.1 mm, more preferably not more than 3.8 mm, and even more preferably not more than 3.5 mm. If this value is too large, the ball may have an inadequate rebound, as a result of which a sufficient distance may not be achieved, and the ball may have a poor durability to cracking under repeated impact. On the other hand, if this value is too large, the feel on full shots may become hard and the spin rate may become too high, as a result of which a sufficient distance may not be achieved.

The material making up the core having the above desired properties is not subject to any particular limitation. The core may be formed using a rubber composition containing, for example, a co-crosslinking agent, an organic peroxide, an inert filler and an organosulfur compound. It is preferable to use a polybutadiene as the base rubber of this rubber composition.

It is desirable for the polybutadiene to have a cis-1,4 bond content on the polymer chain of at least 60 wt %, preferably at least 80 wt %, more preferably at least 90 wt %, and most preferably at least 95 wt %. Too low a cis-1,4 bond content among the bonds on the molecule may result in a lower resilience.

Also, the polybutadiene has a 1,2-vinyl bond content on the polymer chain of generally not more than 2%, preferably not more than 1.7%, and more preferably not more than 1.5%. Too high a 1,2-vinyl bond content may result in a lower resilience.

To obtain a molded and vulcanized rubber composition which has a high resilience and thus increases the distance traveled by the ball, the above polybutadiene is preferably one synthesized with a rare-earth catalyst or a Group VIII metal compound catalyst. Polybutadiene synthesized with a rare-earth catalyst is especially preferred.

Such rare-earth catalysts are not subject to any particular limitation. Exemplary rare-earth catalysts include those made up of a combination of a lanthanide series rare-earth compound with an organoaluminum compound, an alumoxane, a halogen-bearing compound and an optional Lewis base.

Examples of suitable lanthanide series rare-earth compounds include halides, carboxylates, alcoholates, thioalcoholates and amides of atomic number 57 to 71 metals.

In particular, the use of a neodymium catalyst in which a neodymium compound serves as the lanthanide series rare-earth compound is advantageous because it enables a polybutadiene rubber having a high cis-1,4 bond content and a low 1,2-vinyl bond content to be obtained at an excellent polymerization activity. Preferred examples of such rare-earth catalysts include those mentioned in JP-A 11-35633, JP-A 11-164912 and JP-A 2002-293996.

To increase the resilience, it is preferable for the polybutadiene synthesized using the lanthanide series rare-earth compound catalyst to account for at least 10 wt %, preferably at least 20 wt %, and more preferably at least 40 wt %, of the rubber components.

Rubber components other than the above-described polybutadiene may be included in the base rubber insofar as the objects of the invention are attainable. Illustrative examples of rubber components other than the above-described polybutadiene include other polybutadienes, and other diene rubbers, such as styrene-butadiene rubber, natural rubber, isoprene rubber and ethylene-propylene-diene rubber.

Examples of co-crosslinking agents include unsaturated carboxylic acids and the metal salts of unsaturated carboxylic acids.

Specific examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.

The metal salts of unsaturated carboxylic acids, while not subject to any particular limitation, are exemplified by the above-mentioned unsaturated carboxylic acids neutralized with a desired metal ion. Specific examples include the zinc salts and the magnesium salts of methacrylic acid and acrylic acid. The use of zinc acrylate is especially preferred.

The amount of unsaturated carboxylic acid and/or metal salt thereof included per 100 parts by weight of the base rubber is set to generally at least 10 parts by weight, preferably at least 15 parts by weight, and more preferably at least 20 parts by weight. The upper limit is set to generally not more than 60 parts by weight, preferably not more than 50 parts by weight, more preferably not more than 45 parts by weight, and most preferably not more than 40 parts by weight. Too much may make the core too hard, giving the ball an unpleasant feel on impact, whereas too little may lower the rebound.

The organic peroxide may be a commercially available product, suitable examples of which include Percumyl D (available from NOF Corporation), Perhexa 3M (NOF Corporation), and Luperco 231XL (Atochem Co.). These may be used singly or as a combination of two or more thereof.

The amount of organic peroxide included per 100 parts by weight of the base rubber is set to generally at least 0.1 part by weight, preferably at least 0.3 part by weight, more preferably at least 0.5 part by weight, and most preferably at least 0.7 part by weight. The upper limit is set to generally not more than 5 parts by weight, preferably not more than 4 parts by weight, more preferably not more than 3 parts by weight, and most preferably not more than 2 parts by weight. Too much or too little organic peroxide may make it impossible to achieve a ball having a good feel, durability and rebound.

Examples of preferred inert fillers include zinc oxide, barium sulfate and calcium carbonate. These may be used singly or as a combination of two or more thereof.

The amount of inert filler included per 100 parts by weight of the base rubber is set to generally at least 1 part by weight, and preferably at least 5 parts by weight. The upper limit is set to generally not more than 50 parts by weight, preferably not more than 40 parts by weight, more preferably not more than 30 parts by weight, and most preferably not more than 20 parts by weight. Too much or too little inert filler may make it impossible to achieve a proper weight and a good rebound.

In addition, an antioxidant may be included if necessary. Illustrative examples of suitable commercial antioxidants include Nocrac NS-6, Nocrac NS-30 (both available from Ouchi Shinko Chemical Industry Co., Ltd.), and Yoshinox 425 (Yoshitomi Pharmaceutical Industries, Ltd.). These may be used singly or as a combination of two or more thereof.

The amount of antioxidant included per 100 parts by weight of the base rubber is preferably at least 0.05 part by weight, more preferably at least 0.1 part by weight, and most preferably at least 0.2 part by weight. The upper limit is generally not more than 3 parts by weight, preferably not more than 2 parts by weight, more preferably not more than 1 part by weight, and most preferably not more than 0.5 part by weight. Too much or too little antioxidant may make it impossible to achieve a good rebound and durability.

To enhance the rebound of the golf ball and increase its initial velocity, it is preferable to include an organosulfur compound in the above core. Here, it is recommended that, for example, a thiophenol, a thionaphthol, a halogenated thiophenol, or a metal salt thereof be included as the organosulfur compound. Specific examples include pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, p-chlorothiophenol, the zinc salt of pentachlorothiophenol, and also diphenylpolysulfides, dibenzylpolysulfides, dibenzoylpolysulfides, dibenzothiazoylpolysulfides and dithiobenzoylpolysulfides having 2 to 4 sulfurs. The use of diphenyldisulfide or of the zinc salt of pentachlorothiophenol is especially preferred.

The amount of organosulfur compound included per 100 parts by weight of the base rubber is preferably at least 0.05 part by weight, more preferably at least 0.1 part by weight, and even more preferably at least 0.2 part by weight. If the amount included is too small, a rebound-improving effect is unlikely to occur. The upper limit in the amount of organosulfur compound included per 100 parts by weight of the base rubber is preferably not more than 5 parts by weight, more preferably not more than 3 parts by weight, and even more preferably not more than 2.5 parts by weight. If the amount included is too large, a further rebound-improving effect (especially on shots with a W#1) is unlikely to occur, or the core may become too soft, resulting in a poor feel.

It is desirable to produce the core by using a conventional mixer, such as a Banbury mixer or a roll mill, to masticate the core composition obtained by blending the above ingredients, then compression-molding or injection-molding the composition in a core-forming mold. The molded body obtained is then suitably heated and cured at a temperature sufficient for the crosslinking agent and co-crosslinking agent to act, generally from about 130° C. to about 170° C., and especially from 150° C. to 160° C., for a period of from 10 to 40 minutes, and especially from 12 to 20 minutes, so as to give a core having a given hardness profile.

Next, the intermediate layer is described.

The intermediate layer has a material hardness, expressed as the Shore D hardness (measured with a type D durometer in general accordance with ASTM D 2240; the same applies below), which is at least 42, preferably at least 43, and more preferably at least 44. The upper limit is not more than 76, preferably not more than 73, more preferably not more than 70, even more preferably 68, and most preferably 66. If the intermediate layer is too soft, the ball may have too much spin receptivity on full shots, as a result of which a good distance may not be achieved. On the other hand, if the intermediate layer is too hard, the durability of the ball to cracking on repeated impact may worsen and the ball may have too hard a feel when played with a putter or on short approach shots.

The intermediate layer has a thickness which is preferably at least 0.7 mm, more preferably at least 0.9 mm, and even more preferably at least 1.1 mm. The upper limit is preferably not more than 1.7 mm, more preferably not more than 1.5 mm, and even more preferably not more than 1.3 mm. If the intermediate layer is thicker than the above range, the spin rate-lowering effect on shots with a W#1 may be inadequate, as a result of which a good distance may not be achieved. On the other hand, if the intermediate layer is too thin, the durability of the ball to cracking on repeated impact and the low-temperature durability may worsen.

The intermediate layer material is not subject to any particular limitation, although use may be made of various types of thermoplastic resins or thermoplastic elastomers. The use of a resin composition composed primarily of an ionomer is especially preferred. Specifically, a resin composition obtained by the mixture of a zinc ion-neutralized ionomer with a sodium ion-neutralized ionomer is desirable. The mixing ratio I/II between the zinc ion-neutralized ionomer (I) and the sodium ion-neutralized ionomer (II), expressed as a weight ratio, is preferably from 25/75 to 75/25, more preferably from 35/65 to 65/35, and even more preferably from 45/55 to 55/45. Outside of this range, when a zinc-neutralized ionomer and a sodium-neutralized ionomer are mixed, the rebound of the ball as a whole may become too low, as a result of which the desired distance may not be obtained, or the durability to cracking under repeated impact at normal temperatures may worsen, in addition to which the durability to cracking at low (sub-zero Celsius) temperatures may worsen.

Also, in cases where, as is subsequently described, polyurethane is used as the cover material, it is desirable to abrade the surface of the intermediate layer so as to increase adhesion with the urethane cover material. In addition, it is desirable to apply a primer to the surface of the intermediate layer following such abrasion treatment or to add an adhesion-reinforcing agent to the intermediate layer material.

Ratio of Core Diameter to Intermediate Layer Thickness

In this invention, it is desirable to optimize the ratio of the core diameter to the intermediate cover thickness within a given range. Specifically, the ratio c/a of the core diameter (c) to the intermediate layer thickness (a) is from 23 to 38, preferably from 24 to 38, and more preferably from 25 to 38. If this value is too small, the feel on impact may harden or the rebound may be inadequate, as a result of which a sufficient distance may not be achieved. On the other hand, if this value is too large, the feel on impact may harden or the spin receptivity may be excessive, as a result of which a sufficient distance may not be achieved.

Next, the cover used in the invention has a material hardness, expressed as the Shore D hardness, of at least 41, preferably at least 42, and more preferably at least 43. The upper limit is preferably not more than 69, more preferably not more than 66, and even more preferably not more than 63. If the cover is softer than the above range, the ball may have too much spin receptivity, as a result of which a good distance may not be achieved on shots with a W#1. On the other hand, if the cover is harder than the above range, the ball may lack spin receptivity on approach shots, as a result of which the controllability may be inadequate even for professional golfers and skilled amateurs.

The cover has a thickness which is preferably at least 0.4 mm, more preferably at least 0.6 mm, and even more preferably at least 0.8 mm. The upper limit is not more than 1.4 mm, preferably not more than 1.2 mm, and more preferably not more than 1.0 mm. If the cover is thicker than the above range, the ball may have an excessive spin receptivity on shots with a W#1, as a result of which a good distance may not be obtained. On the other hand, if the cover is thinner than the above range, the ball may have too little spin receptivity in the short game, and may thus have a poor controllability.

The cover material is not subject to any particular limitation, although use may be made of various types of thermoplastic resins and thermoplastic elastomers, with formation typically being carried out using polyurethane in particular as the primary material. From the standpoint of productivity, a thermoplastic polyurethane elastomer is preferred.

Specifically, it is preferable to use a specific thermoplastic polyurethane composition composed primarily of (A) a thermoplastic polyurethane and (B) a polyisocyanate compound. This resin blend is described below.

To fully and effectively achieve the objects of the invention, a necessary and sufficient amount of unreacted isocyanate groups should be present within the cover resin material. Specifically, it is recommended that the total weight of components A and B combined be preferably at least 60%, and more preferably at least 70%, of the overall weight of the cover layer. Above components A and B are described in detail below.

In describing the thermoplastic polyurethane (A), the structure of this thermoplastic polyurethane includes soft segments composed of a polymeric polyol that is a long-chain polyol (polymeric glycol), and hard segments composed of a chain extender and a polyisocyanate compound. Here, the long-chain polyol used as a starting material is not subject to any particular limitation, and may be any that is used in the prior art relating to thermoplastic polyurethanes. Exemplary long-chain polyols include polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. These long-chain polyols may be used singly or as combinations of two or more thereof. Of the long-chain polyols mentioned here, polyether polyols are preferred because they enable the synthesis of thermoplastic polyurethanes having a high rebound resilience and excellent low-temperature properties.

Illustrative examples of the above polyether polyol include poly(ethylene glycol), poly(propylene glycol), poly(tetramethylene glycol) and poly(methyltetramethylene glycol) obtained by the ring-opening polymerization of cyclic ethers. The polyether polyol may be used singly or as a combination of two or more thereof. Of the above, poly(tetramethylene glycol) and/or poly(methyltetramethylene glycol) are preferred.

It is preferable for these long-chain polyols to have a number-average molecular weight in a range of 1,500 to 5,000. By using a long-chain polyol having a number-average molecular weight within this range, golf balls made with a thermoplastic polyurethane composition having excellent properties such as resilience and manufacturability can be reliably obtained. The number-average molecular weight of the long-chain polyol is more preferably in a range of 1,700 to 4,000, and even more preferably in a range of 1,900 to 3,000.

The number-average molecular weight of the long-chain polyol refers here to the number-average molecular weight computed based on the hydroxyl number measured in accordance with JIS K-1557.

Chain extenders that may be suitably used include those employed in the prior art relating to thermoplastic polyurethanes. For example, low-molecular-weight compounds which have a molecular weight of 400 or less and bear on the molecule two or more active hydrogen atoms capable of reacting with isocyanate groups are preferred. Illustrative, non-limiting, examples of the chain extender include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. Of these chain extenders, aliphatic diols having 2 to 12 carbons are preferred, and 1,4-butylene glycol is more preferred.

The polyisocyanate compound is not subject to any particular limitation; preferred use may be made of one that is used in the prior art relating to thermoplastic polyurethanes. Specific examples include one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. Depending on the type of isocyanate used, the crosslinking reaction during injection molding may be difficult to control. In the practice of the invention, to provide a balance between stability at the time of production and the properties that are manifested, it is most preferable to use 4,4′-diphenylmethane diisocyanate, which is an aromatic diisocyanate.

It is most preferable for the thermoplastic polyurethane serving as above component A to be a thermoplastic polyurethane synthesized using a polyether polyol as the long-chain polyol, using an aliphatic diol as the chain extender, and using an aromatic diisocyanate as the polyisocyanate compound. It is desirable, though not essential, for the polyether polyol to be a polytetramethylene glycol having a number-average molecular weight of at least 1,900, for the chain extender to be 1,4-butylene glycol, and for the aromatic diisocyanate to be 4,4′-diphenylmethane diisocyanate.

The mixing ratio of active hydrogen atoms to isocyanate groups in the above polyurethane-forming reaction may be adjusted within a desirable range so as to make it possible to obtain a golf ball which is composed of a thermoplastic polyurethane composition and has various improved properties, such as rebound, spin performance, scuff resistance and manufacturability. Specifically, in preparing a thermoplastic polyurethane by reacting the above long-chain polyol, polyisocyanate compound and chain extender, it is desirable to use the respective components in proportions such that the amount of isocyanate groups on the polyisocyanate compound per mole of active hydrogen atoms on the long-chain polyol and the chain extender is from 0.95 to 1.05 moles.

No particular limitation is imposed on the method of preparing the thermoplastic polyurethane used as component A. Production may be carried out by either a prepolymer process or a one-shot process which uses a long-chain polyol, a chain extender and a polyisocyanate compound and employs a known urethane-forming reaction. Of these, a process in which melt polymerization is carried out in a substantially solvent-free state is preferred. Production by continuous melt polymerization using a multiple screw extruder is especially preferred.

It is also possible to use a commercially available product as the thermoplastic polyurethane serving as component A. Illustrative examples include Pandex T8295, Pandex T8290 and Pandex T8260 (all available from DIC Bayer Polymer, Ltd.).

Next, concerning the polyisocyanate compound used as component B, it is necessary that, in a single resin blend (in the form of pellets) prior to molding, all the isocyanate groups in at least some portion of the polyisocyanate compound remain in an unreacted state. That is, some polyisocyanate compound in which the isocyanate groups on the molecule remain in a completely free state must be present in the resin blend; such a polyisocyanate compound may be present together with polyisocyanate compound in which only one end of the molecule is in a free state.

Various types of isocyanates may be employed without particular limitation as the polyisocyanate compound. Illustrative examples include one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate and dimer acid diisocyanate. Of the above group of isocyanates, the use of 4,4′-diphenylmethane diisocyanate, dicyclohexylmethane diisocyanate and isophorone diisocyanate is preferable in terms of the balance between the influence on processability of, e.g., the rise in viscosity accompanying the reaction with the thermoplastic polyurethane serving as component A and the physical properties of the resulting golf ball cover material.

In the invention, although not an essential constituent, a thermoplastic elastomer other than the above-described thermoplastic polyurethane may be included as component C together with components A and B. Including this component C in the above resin blend enables the flow properties of the resin blend to be further improved and enables improvements to be made in various properties required of golf ball cover materials, such as resilience and scuff resistance.

In addition to the above resin components, various additives may be optionally included in the above-described cover-forming resin material. Examples of such additives include pigments, dispersants, antioxidants, ultraviolet absorbers, ultraviolet stabilizers, parting agents, plasticizers and inorganic fillers (e.g., zinc oxide, barium sulfate, titanium dioxide).

Ratio of Intermediate Layer Thickness to Cover Thickness

In the invention, it is desirable to optimize the ratio of the intermediate layer thickness to the cover thickness within a given range. Specifically, the ratio a/b of the intermediate layer thickness (a) to the cover thickness (b) is from 0.7 to 1.9, preferably from 0.8 to 1.9, and more preferably from 0.9 to 1.9. If this value is too small, the feel on impact may harden or the spin receptivity may be excessive, as a result of which a sufficient distance may not be achieved. On the other hand, if this value is too large, the rebound may be inadequate, as a result of which a sufficient distance may not be achieved.

Relationship Between Intermediate Layer Material Hardness, Cover Material Hardness and Core Surface Hardness

An essential condition in the invention is that the intermediate layer material hardness, the cover material hardness and the core surface hardness satisfy the following relationship therebetween:

cover material hardness<intermediate layer material hardness>core surface hardness.

By setting the hardnesses of the respective layers so as to maintain this relationship, it is possible to further enhance the flight performance and to obtain a good, solid feel on impact.

In the invention, a plurality of dimples are formed on the surface of the cover. The number of dimples arranged on the cover surface, although not subject to any particular limitation, is preferably at least 250, more preferably at least 300, and even more preferably at least 318. The upper limit is preferably not more than 350, and more preferably not more than 328. If the number of dimples is larger than the above range, the ball trajectory may become lower, possibly decreasing the distance traveled by the ball. On the other hand, if the number of dimples is smaller than the above range, the ball trajectory may become higher, as a result of which an increased distance may not be achieved.

The shapes of the dimples are not subject to any particular limitation; any one type, or combination of two or more types, from among, e.g., circular shapes, various polygonal shapes, dewdrop shapes and oval shapes may be suitably selected. For example, in cases where circular dimples are used, dimples having a diameter of from 2.5 to 6.0 mm may be suitably selected. As for the number of dimple types, by suitably using from at least 3 to 5 or more types, and preferably 5 or more types, it is possible to cover the spherical surface with dimples in a manner that is uniform and well-balanced.

The types of dimples are not subject to any particular limitation. The dimples may be suitably arranged in a spherical polyhedral configuration that is based on a repeated pattern of unit polygons, such as unit triangles and unit pentagons. Moreover, it is possible to use all the dimples at slightly varying diameters. In such a case, the number of dimple types may be set to 20 or more. To fully manifest the aerodynamic properties, it is desirable for the sum of the dimple surface areas, each defined by the border of the flat plane circumscribed by the edge of a dimple, when expressed as a ratio with respect to the spherical surface of the ball were it to be free of dimples, to be at least 75%.

In addition, it is preferable for the ball to include at least six small dimples having a diameter of 3.0 mm or less, with the number of such dimples being more preferably in a range of from 6 to 30. In the invention, by intermingling large and small dimples so as to increase the surface coverage, it is possible to achieve the effect of, in the first half of the ball trajectory, making the coefficient of lift (CL) larger and making the coefficient of drag (CD) smaller. Making the drag or the coefficient of drag CD smaller is not by itself very effective for increasing the distance traveled on a shot. Simply reducing the coefficient of drag does extend the position of the highest point on the trajectory, but tends to result in a loss in carry because the ball drops on account of insufficient lift in the low-velocity portion of the trajectory after the highest point. Accordingly, in the multi-piece solid golf ball of the invention, it is preferable for the ball to have a coefficient of drag CD of 0.225 or below when the Reynolds number is 180,000 and the spin rate is 2,520 rpm immediately after being hit and launched, and for the ball, when hit, to have a coefficient of lift CL at a Reynolds number of 70,000 and a spin rate of 2000 rpm which remains at least 60% of the coefficient of lift CL at a Reynolds number of 80,000 and a spin rate of 2,000 rpm. A Reynolds number of 180,000 immediately after the ball is hit and launched corresponds to a ball velocity of about 66 m/s, and Reynolds numbers of 80,000 and 70,000 correspond to velocities of about 30 m/s and 26 m/s, respectively.

The golf ball of the invention can be manufactured so as to conform with the Rules of Golf for competitive play, with the ball preferably being of a diameter that will not pass through a ring having an inside diameter of 42.672 mm, but is not more than 42.80 mm, and of a weight that is generally from 45.0 to 45.93 g.

Ball Deflection

In the present invention, letting the ball deflection (mm) when compressed under a final load of 490 N (50 kgf) from an initial load state of 98 N (10 kgf) be A and letting the ball deflection (mm) when compressed under a final load of 5,880 N (600 kgf) from an initial load state of 98 N (10 kgf) be B, the value of B/A×100 is from 830 to 930, preferably from 840 to 920, and more preferably from 830 to 910. If the above value is too small, the ball may have too much spin receptivity on shots with a driver, as a result of which a sufficient distance may not be achieved. On the other hand, if the above value is too large, the initial velocity on shots with a driver may be inadequate, as a result of which a sufficient distance may not be achieved.

The ball deflection B, although not subject to any particular limitation, is preferably from 7.0 to 10.0 mm.

The ball has a deflection (mm), when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf), which, although not subject to any particular limitation, is preferably at least 2.1 mm, more preferably at least 2.2 mm, and even more preferably at least 2.3 mm. The upper limit, although not subject to any particular limitation, is preferably not more than 2.9 mm, more preferably not more than 2.8 mm, and even more preferably not more than 2.7 mm.

As described above, the multi-piece solid golf ball of the invention has an improved flight performance and a good, solid feel on impact.

EXAMPLES

Examples of the invention and Comparative Examples are given below by way of illustration, and not by way of limitation.

Examples 1 to 3, Comparative Examples 1 to 3

Rubber compositions were formulated as shown in Table 1 below, then molded and vulcanized at 155° C. for 13 minutes to form solid cores.

TABLE 1 Example Comparative Example (parts by weight) 1 2 3 1 2 3 Polybutadiene A 80 80 80 80 80 80 Polybutadiene B 20 20 20 20 20 20 Peroxide 1.2 1.2 1.2 1.2 1.2 1.2 Barium sulfate 13.3 14.6 16.1 22.0 13.8 14.0 Zinc oxide 4.0 4.0 4.0 4.0 4.0 4.0 Antioxidant 0.1 0.1 0.1 0.1 0.1 0.1 Zinc acrylate 31.7 30.5 29.3 25.9 34.5 33.5 Zinc stearate — — — — — 5 Sulfur — — — — — 0.12 Zinc salt of 0.2 0.2 0.2 0.2 0.2 0.2 pentachlorothiophenol

Details on the above core materials are given below. Numbers in the table represent parts by weight.

-   Polybutadiene A: Available under the trade name “BR 730” from JSR     Corporation. -   Polybutadiene B: Available under the trade name “BR 01” from JSR     Corporation. -   Peroxide: A mixture of 1,1-di(t-butylperoxy)cyclohexane and silica,     available under the trade name “Perhexa C-40” from NOF Corporation. -   Antioxidant: 2,2′-Methylenebis(4-methyl-6-t-butylphenol), available     under the trade name “Nocrac NS-6” from Ouchi Shinko Chemical     Industry Co., Ltd. -   Barium sulfate: Available under the trade name “Precipitated Barium     #300” from Sakai Chemical Co., Ltd.

Formation of Intermediate Layer and Cover

Next, using the No. 1, No. 2 or No. 4 formulation shown in Table 2, an intermediate layer (one or two layers) was injection-molded over the core obtained above, thereby producing an intermediate sphere.

Next, the respective starting material (units: parts by weight) of No. 3 shown in Table 2 were mixed under a nitrogen atmosphere in a twin-screw extruder, thereby giving a cover-forming resin blend. This resin blend was in the form of pellets having a length of 3 mm and a diameter of 1 to 2 mm.

The intermediate sphere was placed within an injection mold and the cover material was injection-molded over this sphere, thereby giving multi-piece solid golf balls in Examples 1 to 3 and Comparative Examples 1 to 3. To measure the cover properties, the cover material was injection-molded as a 2 mm thick sheet, which was annealed at 100° C. for 8 hours, then held at room temperature for one week and subsequently furnished for testing.

The dimples shown in FIG. 2 were formed at this time on the cover surface in the respective examples and comparative examples. Details on the dimples are shown in Table 3.

TABLE 2 (parts by weight) No. 1 No. 2 No. 3 No. 4 Himilan 1605 50 Himilan 1557 15 Himilan 1706 35 Surlyn 8120 75 Dynaron 6100P 25 AN4319 20 AN4221C 80 Magnesium stearate 60 Magnesium oxide 1.7 Behenic acid 20 Calcium hydroxide 2.3 Calcium stearate 0.15 Zinc stearate 0.15 Trimethylolpropane 1.1 Polytail H 2 T-8295 T-8290 75 T-8293 25 Titanium oxide 3.5 Polyethylene 1 Isocyanate compound 7.5

The above trade names are explained below.

Trade names for the chief materials shown in the table are as follows.

-   Himilan: Ionomers available from DuPont-Mitsui Polychemicals Co.,     Ltd. -   Surlyn: An ionomer available from E.I. DuPont de Nemours & Co. -   Dynaron E6100P: A hydrogenated polymer available from JSR     Corporation. -   AN4319, AN4221C: “Nucrel,” available from DuPont-Mitsui     Polychemicals Co., Ltd. -   Magnesium stearate: Available under the trade name “Magnesium     Stearate G” from NOF Corporation. -   Magnesium oxide: “Kyowamag MF150,” available from Kyowa Chemical     Industry Co., Ltd. -   Behenic acid: NAA222-S (beads), available from NOF Corporation. -   Calcium hydroxide: CLS-B, available from Shiraishi Kogyo. -   T-8925, T-8290, T-8283: MDI-PTMG type thermoplastic polyurethanes     available under the trade name “Pandex” from DIC Bayer Polymer. -   Polyethylene wax: Available under the trade name “Sanwax 161P” from     Sanyo Chemical Industries, Ltd. -   Isocyanate compound: 4,4′-Diphenylmethane diisocyanate

TABLE 3 No. Number of dimples Diameter (mm) 1 18 4.7 2 258 4.5 3 18 3.7 4 26 3.4 5 6 2.9 Dimple types 5 types Number of dimples 326 SR (%) 80 Low-velocity CL ratio (%) 82

Dimple Definitions

-   Diameter: Diameter of flat plane circumscribed by edge of dimple. -   Depth: Maximum depth of dimple from flat plane circumscribed by edge     of dimple. -   V_(o): Spatial volume of dimple below flat plane circumscribed by     dimple edge, divided by volume of cylinder whose base is the flat     plane and whose height is the maximum depth of dimple from the base. -   SR: Sum of individual dimple surface areas, each defined by the flat     plane circumscribed by the edge of a dimple, as a percentage of     surface area of ball sphere were it to have no dimples thereon.     (units: %)

Aerodynamic Properties (Low-Velocity CL Ratio, High-Velocity CD Value)

The low-velocity CL ratio was obtained by calculating the ratio of the coefficient of lift CL of a ball on its trajectory just after launch using an Ultra Ball Launcher (UBL) at a Reynolds number of 70,000 and a spin rate of 2,000 rpm with respect to the coefficient of lift CL of a ball launched at a Reynolds number of 80,000 and a spin rate of 2,000 rpm. Similarly, the high-velocity CD value was obtained by calculating the coefficient of drag when the ball was launched at a Reynolds number or 180,000 and a spin rate of 2,520 rpm.

The UBL is a device which includes two pairs of drums, one on top and one on the bottom. The drums are turned by belts that extend across the two top drums and across the two bottom drums. The UBL inserts a golf ball between the turning drums and thereby launches the golf ball under the desired conditions. This device is manufactured by Automated Design Corporation.

The various golf balls obtained were tested and evaluated by the methods described below with regard to properties of the various layers, such as thickness, hardness and deflection, and also flight performance and feel. The results are shown in Tables 4 and 5. All measurements were carried out in a 23° C. atmosphere.

(1) Deflection (mm) of Core and Intermediate Layer-Covered Sphere

The core or the intermediate layer-covered sphere was compressed at a temperature of 23±1° C. and a speed of 50 mm/min, and the amount of deflection (mm) incurred by the core or sphere when subjected to a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) was measured. The average for 10 specimens was determined.

(2) Core Surface Hardness

The durometer indenter was set substantially perpendicular to the spherical surface of the core and the Shore D hardness (the hardness based on a type D durometer in accordance with ASTM-2240) was determined. The average of the two measurements was used as the core surface hardness.

(3) Material Hardnesses of Intermediate Layer and Cover (Hardness of Sheet-Type Molding)

The respective layer-forming materials were molded into sheets having a thickness of about 2 mm and held for two weeks at 23° C., following which the hardnesses were measured with a type D durometer (i.e., “Shore D hardness”) in accordance with ASTM-2240.

(4) Ball Deflection (Deflection Under 50 kgf Loading, 130 kgf Loading, and 600 kgf Loading)

The ball was compressed at a temperature of 23±1° C. and a speed of 500 mm/min, and the amount of deflection (mm) incurred by the ball when subjected to a final load of 5,880 N (600 kgf) from an initial load state of 98 N (10 kgf) was measured. The average for 10 specimens was determined.

Also, the ball was compressed at a temperature of 23±1° C. and a speed of 500 mm/min, and the amount of deflection (mm) incurred by the ball when subjected to a final load of 490 N (50 kgf) from an initial load state of 98 N (10 kgf) was measured. The average for 10 specimens was determined.

(5) Flight Performance on Shots with Driver

The distance traveled by the ball when hit at a head speed (HS) of 50 m/s with a driver (abbreviated below as “W#1”; TourStage GR (2010 model), manufactured by Bridgestone Sports Co., Ltd.; loft angle,)10.5°) mounted on a golf swing robot was measured. The results were rated according to the criteria shown below. The spin rate was the value measured for the ball, using an apparatus for measuring initial conditions, immediately after the ball was hit in the same way as described above.

Good: Carry was 245 m or more

NG: Carry was less than 245 m

(6) Feel

Golfers who value distance and have a head speed (HS) of 48 m/s hit the ball with a driver (W#1) and carried out sensory evaluations according to the criteria shown below.

The driver (W#1) used was the same as in (5) above: a TourStage GR (2010 model), manufactured by Bridgestone Sports Co., Ltd., and having a loft angle of 10.5°.

Good: At least seven out of ten golfers thought the ball had a good feel.

NG: Three or fewer out of ten golfers thought the ball had a good feel.

(A “good feel” refers to a solid feel on impact which leaves the impression that the ball will fly far. A feel that is too soft or too hard is regarded as a “poor feel.”)

TABLE 4 Example Comparative Example 1 2 3 1 2 3 Dimples Total number 326 326 326 326 326 326 Low-velocity CL ratio (%) 82 82 82 82 82 82 SR value (%) 80 80 80 80 80 80 Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 Deflection 1.0 1.0 1.0 0.9 1.0 1.0 under 50 kg loading (A) Deflection 2.5 2.5 2.5 2.6 2.4 2.5 under 130 kg loading Deflection 8.3 8.5 8.7 8.8 8.1 8.5 under 600 kg loading (B) Cover Material (type) No. 3 No. 3 No. 3 No. 3 No. 3 No. 3 Shore D hardness 56.5 56.5 56.5 56.5 56.5 56.5 Thickness (mm) (b) 0.8 0.8 0.8 0.8 0.3 0.8 Intermediate Material (type) No. 1 No. 1 No. 1 No. 1 No. 1 No. 1 layer Shore D hardness 62 62 62 62 62 62 (1) Thickness (mm) (a) 1.1 1.2 1.4 1.2 1.1 1.2 Diameter of intermediate 41.1 41.1 41.1 41.1 42.1 41.1 layer-covered sphere (mm) Deflection 2.8 2.8 2.8 2.8 2.45 2.8 under 130 kg loading (mm) Intermediate Material (type) No. 4 layer Shore D hardness 55 (2) Thickness (mm) (a) 1.1 Diameter of intermediate 38.8 layer-covered sphere (mm) Deflection 3.2 under 130 kg loading (mm) Core Diameter (mm) (c) 39.0 38.7 38.4 36.7 40 38.7 Deflection 2.95 3.10 3.25 3.56 2.70 3.10 under 130 kg loading (mm) Shore D hardness 58 57 56 53 60 59 at surface (1) Shore D hardness 46 44 43 40 48 41 at center (2) (1) − (2) 12 13 13 13 12 18 Ratio of intermediate layer 1.3 1.5 1.7 2.9 3.5 1.5 thickness to cover thickness (a/b) Ratio of core diameter to 37.1 32.3 28.4 15.9 38.1 32.3 intermediate layer thickness (c/a) Deflection ratio: 600 kg 860 886 899 937 838 886 loading to 50 kg loading (B/A × 100)

TABLE 5 Example Comparative Example 1 2 3 1 2 3 Flight Initial velocity (m/s) 72.3 72.0 71.7 71.4 72.1 71.0 (W#1) Spin rate (rpm) 2,500 2,416 2,350 2,461 2,684 2,380 Carry (m) 246 247 245 242 244 241 Rating good good good NG NG NG Feel (W#1) good good good NG NG NG

The results in Table 5 show that the respective comparative examples were inferior to the present invention (working examples) in the following ways.

In Comparative Example 1, the ratio of the core diameter to the intermediate layer thickness was small and the initial velocity on a shot with a W#1 was insufficient, as a result of which a good carry was not achieved. In addition, the feel of the ball when hit with a W#1 was too soft and left something to be desired.

In Comparative Example 2, the ratio of the core diameter to the intermediate layer thickness was large and the spin rate-lowering effect on shots with a W#1 was inadequate, as a result of which a good carry was not achieved. Moreover, the feel of the ball when hit with a W#1 was too hard.

In Comparative Example 3, a Shore D hardness at the core surface minus a Shore D hardness at the core center is too great, the ball on full shots deformed largely to yield an initial velocity loss of the ball, as a result of which a good carry was not achieved. Also, the ball on full shots deformed largely, a result of which a sufficient good feel was not obtained and the feel was not endowed with an expectation of good flight distance. 

1. A multi-piece solid golf ball comprising a core, at least one intermediate layer encasing the core, and a cover of at least one layer encasing the intermediate layer, wherein the core is formed of a base rubber, the intermediate layer and cover are each formed of a resin material, the intermediate layer has a thickness (a) and the cover has a thickness (b) such that the ratio a/b is from 0.7 to 1.9, the core has a diameter (c) such that the ratio c/a with the intermediate layer thickness (a) is from 23 to 38, the intermediate layer has a material hardness (Shore D) of from 42 to 76, the cover has a material hardness (Shore D) of from 41 to 69, and the ball satisfies the following relationship: cover material hardness<intermediate layer material hardness>core surface hardness, and wherein a Shore D hardness at the core surface minus a Shore D hardness at the core center is not more than 17 units.
 2. The multi-piece solid golf ball of claim 1 wherein, letting the ball deflection (mm) when compressed under a final load of 490 N (50 kgf) from an initial load state of 98 N (10 kgf) be A and letting the ball deflection (mm) when compressed under a final load of 5,880 N (600 kgf) from an initial load state of 98 N (10 kgf) be B, the value of B/A×100 is from 830 to
 930. 3. The multi-piece solid golf ball of claim 1, wherein the ball deflection B is from 7.0 to 10.0 mm, and the core deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) is from 2.1 to 4.1 mm.
 4. The multi-piece solid golf ball of claim 1, wherein the resin material of the cover is formed by injection molding a single resin blend composed primarily of (A) a thermoplastic polyurethane and (B) a polyisocyanate compound, which resin blend includes a polyisocyanate compound in at least some portion of which all the isocyanate groups on the molecule remain in an unreacted state.
 5. The multi-piece solid golf ball of claim 1, wherein a plurality of dimples are formed on a surface of the ball, the total number of dimples is from 250 to 350, the dimples have a surface coverage (SR) of at least 75%, and the ball, when hit, has a coefficient of lift CL at a Reynolds number of 70,000 and a spin rate of 2,000 rpm which is at least 60% of the coefficient of lift CL at a Reynolds number of 80,000 and a spin rate of 2,000 rpm.
 6. The multi-piece solid golf ball of claim 5 which uses at least five types of dimples of differing diameter and/or depth, and which includes from 6 to 30 small dimples having a diameter of 3.0 mm or less. 